This invention relates generally to devices for recording data on a record medium, and, more particularly, to devices for detecting defects in such a medium that would inhibit such recording.
Devices of this particular type are useful in systems for recording data, especially digital data, on magnetic or optical media, wherein defects in the magnetic material or optical surface can inhibit the recording of data thereon. It has been found desirable for the systems to include means for verifying that the desired data has been properly recorded or, alternatively, means for detecting defective regions on the recording medium prior to the recording of data thereon, so that such defective regions can be avoided in the recording process.
Systems in which the data is verified after being recorded are normally classified as "read-after-write" or "read-while-write" systems. In a system typical of this type, for recording data on an optical disc, the data is recorded in a series of circular and concentrically arranged tracks by an intensity-modulated write beam of light that is focused onto the disc as the disc is rotated at a prescribed angular velocity, such as a constant angular velocity or a constant linear velocity. Additionally, a read beam of light is directed at the portion of the disc in which data has just been written to produce a reflected beam that is modulated in intensity by the data actually recorded. This reflected beam is detected and compared to the data that was intended to be written, to verify that it was properly recorded on the disc. In the event that a recording error is detected, the incorrect data is appropriately flagged and a prescribed portion of the data is re-written.
One typical read-after-write optical disc recording system is described in an article written by George C. Kenney et al, entitled "An Optical Disk Replaces 25 Mag Tapes", published in IEEE Spectrum, February, 1979. Although such systems are generally effective in producing recording media from which data can be recovered with substantially no errors, they can sometimes make inefficient use of the available recording capacity, because a substantial portion of the data must ordinarily be re-written whenever errors in the recorded data are detected.
In systems that include means for detecting defective regions on the recording medium prior to the writing of data thereon, the entire medium is typically scanned initially and identification flags are recorded to indicate the location of any defective regions that are detected. After the entire medium has been scanned and any defects thereon flagged, data is then recorded on the non-defective regions of medium. Such systems have not proven entirely satisfactory, however, for a number of reasons, one being that two complete scans of the recording medium are required, one to detect defects and the other to record data. Additionally, in the case of optical discs, wherein the precise locations of the recording tracks are not determined until data is actually recorded, it is sometimes the case that the successive circumferential swaths of the disc that are scanned during the initial scanning to detect defects therein will not correspond exactly with the successive swaths that are scanned when recording. As a result it might be difficult to correlate the two scans and determine exactly when, during the recording scan, a portion of the disc previously determined to defective is being scanned.
It will be appreciated from the foregoing that there is a need for a method and apparatus for recording data on a record medium, wherein defects in the medium can be detected and the data recorded on non-defective portions thereof, without the need for performing two complete scans of the medium, and without the need for re-writing substantial portions of the data. The present invention fulfills this need.